In view of the serious and world-wide increasing pollution of the environment by exhaust emissions of various origin, in particular car exhaust gases, increasingly more rigorous exhaust emission regulations have come into effect over the last decades. In order to comply with these regulations, a variety of catalytic converters for exhaust decontamination, particularly car exhaust decontamination, and fuel cells have been developed.
Since the availability of large catalytically active surfaces is a main issue for heterogenic catalysis, the catalytically active noble metals, such as Pt, Pd, Rh, in fuel cells and catalytic converters for car exhaust gases are usually provided on a washcoat consisting of inorganic oxides, typically γ-Al2O3. In order to achieve and maintain a high-performance catalyst, it is of crucial importance that said catalytically active noble metals are present in a highly disperse form.
The high temperatures occurring during the driving operation of cars can effect an irreversible agglomeration of the catalytically active nanoparticles by means of annealing processes. Thus, already at an operation temperature of 500° C. an increase of the cluster size of the catalytically active nanoparticles—which originally showed a nanodisperse distribution—can be observed. This effect is known as Ostwald Ripening. For example, the thermal aging of a platinum catalyst at temperature of e.g. above 700° C. results in a considerable increase of the mean particle size. The loss of dispersity may amount to more than 80% and, correspondingly, the catalyst may dramatically loose its activity.
Thus, an object of the present invention is to provide a substrate surface comprising disperse, thermally stable metal, preferably noble metal, nanoparticles suitable for use in catalysts, in particular for car exhaust decontamination and fuel cells, which catalysts are thermally stable, highly reactive and involve a very economical use of the costly noble metals.
This object is achieved by providing the method for preparing a nanostructured substrate surface and the use of said nanostructured substrate surface as a catalyst according to the invention.